Upstream: dmu_zfetch_stream_fini leaks refcount

dmu_zfetch_stream_fini() is missing calls to destroy the refcounts,
leaking them and the mutex inside.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12294

module/zfs/dmu_zfetch.c

@@ -159,6 +159,8 @@ static void
 dmu_zfetch_stream_fini(zstream_t *zs)
 {
 	ASSERT(!list_link_active(&zs->zs_node));
+	zfs_refcount_destroy(&zs->zs_callers);
+	zfs_refcount_destroy(&zs->zs_refs);
 	kmem_free(zs, sizeof (*zs));
 }